Issue 52

A. Laureys et alii, Frattura ed Integrità Strutturale, 52 (2020) 113-127; DOI: 10.3221/IGF-ESIS.52.10

a large density of dislocations (10 11 to 10 12 cm -2 ) and residual micro-stresses and thus displays a large hydrogen trapping capacity [50]. Additionally, these regions are surrounded by transformation induced stresses and lattice defects [50], which makes these regions preferential sites for hydrogen accumulation and damage initiation. From the experimental observation, the martensite-martensite interface is the most favorable crack initiation location. The interface is highly strained and most likely a preferential place for hydrogen accumulation. Even though large AlN inclusions were present in the material, these particles were not crack initiation sites. Small initiating cracks were found over the whole cross section of the samples, but initiating cracks were often arrested when propagating frommartensite into ferritic zones (Fig. 11c and d). Plastic deformation of the softer ferritic phase resulted in crack blunting and arrest (Fig. 11d). Such small cracks did not result in the appearance of a blister on the sample surface. Crack propagation was only favorable along the segregation line of the samples, which consisted of a continuous crack sensitive martensitic network (Fig. 12). So even though numerous crack initiation sites are found, only a limited amount of larger hydrogen induced cracks form. This results in a limited number of large blisters at the sample surface, which can be seen in Fig. 7b.

Figure 12: SEM image of hydrogen induced cracking along the segregation line of TRIP-assisted steel. Reprinted with permission from Ref. [40]. Fe-C-Ti alloy Cross section investigation of Fe-C-Ti showed that initiation was related to the intermediate and large titanium precipitates, i.e. Ti(C,N) (Fig. 13). Most often clusters of precipitates were involved [39]. Terasaki et al. [24] also observed a greater HIC sensitivity at clusters of inclusions. Ren et al. [24] stated that Ti containing precipitates can act as nucleation sites for blisters. The precipitates act as hydrogen traps [36, 51], which implies that at these locations an increased amount of hydrogen is present and during continuous hydrogen charging under severe conditions, recombination of hydrogen atoms will occur within the precipitates, leading to fracture of the particles.

Figure 13: SEM image of blister initiation at large precipitates (Ti(C,N)) in Fe-C-Ti alloy. Reprinted with permission from Ref. [39].

A clear interaction of cracks with large Ti-based precipitates was demonstrated as well with EBSD (Fig. 14) [39]. Most likely the hydrogen induced cracking initiated at the large carbides/carbonitrides, due to hydrogen build-up at the interface or in the precipitates.

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